Theoretical Study of Hybrid Guided Modes in a Multilayer Symmetrical Planar Plasmonic Waveguide

We presented a comprehensive theoretical study of the hybrid guided mode in a multilayer symmetrical planar plasmonic waveguide, which is constructed with a thin film metal layer symmetrically sandwiched by three dielectric low/high/low-index layers. The seven-layer planar plasmonic structure can support super long-range plasmonic modes with strong subwavelength confinement in the low-index gap layer. We derived the dispersion equations for the guided mode and characterized the hybrid guided mode based on our derived analytical expressions. We explained how the variations in the thickness of the low-index gap and high-index cladding could change the types of the hybrid mode from strong surface plasmon polariton (SPP)-like mode, to SPP-dielectric waveguide (DW)-like mode, and further to strong-DW-like mode. We also found that by tailoring the geometric dimensions of the waveguide, the plasmonic mode of the multilayer structure can be optimized with the strongest mode confinement at the nanoscale gap. The combination of tight light confinement and long-range propagation length makes the seven-layer plasmonic waveguide an excellent candidate for applications in chip-scale plasmonic integrated circuits. The presented theoretical analysis shall be very useful in the design and optimization of active and passive nanoplasmonic devices.